I think this depends on whether it's a 3.5 or 2.5 inch drive inside. To my knowledge, all external drives with a 3.5 inch drive inside are shuckable and have a standard SATA interface. With the compact drives that have a 2.5 inch drive inside, many will have a native usb interface and no SATA connector.
It makes sense as 3.5" sata drives are used for many many applications so why make something new just for external drives? With 2.5, however there are very few devices that use spinning sata drives in this form factor. It makes a lot more sense to build the USB interface directly on the drive since their main and possibly only application is external drives.
I could be wrong, but this has been my experience.